Lecture 14 - Nervous tissue and the neuromuscular junctions

Question 1

Neurons have the same basic structure

Answer 1

Dendrites
Cell body 
Axon 
Axon hillock 
Axon terminals

Question 2

CNS

Answer 2

brain and spinal cord

Question 3

PNS

Answer 3

peripehral nerves

Question 4

PNS divisions

Answer 4

Afferent division and efferent division

Question 5

Afferent division of PNS

Answer 5

Sensory stimuli - receptors in the skin for example which convey information back to the nervous system

visceral stimuli - internal organs changes and this information gets sent back to the nervous system

Question 6

Efferent division of PNS

Answer 6

Somatic nervous system (voluntary)
- motor neurons that innervate skeletal muscle

Autonomic nervous system (involuntary)

• sympathetic = fight or flight
• parasympathetic = rest and digest

Question 7

Multipolar neurons

Answer 7

Multiple dendrites project from cell body (one axon)

Question 8

Bipolar neurons

Answer 8

Single dendrite opposite from axon

Question 9

Pseudo-unipolar neurons

Answer 9

Axon and dendrite rise from a common stem of the cell body

Question 10

Cellular structure of a neuron

Answer 10

Large nucleus –often large, reflecting metabolic demand
Many mitochondria
Lots of ER, particularly in large neurons. Can be found in dendrites but not axons (protein synthesis in dendrites but not in the axons)
Numerous neurofilaments. Together with microtubules make up the cytoskeleton
Synapses are found at dendrites and also cell body

Question 11

Central nervous system

Answer 11

CNS is macroscopically divided into grey matter (neuron cell bodies, dendrites and axons) and white matter (axons; many myelinated)
Effectively made of myelination that surrounds the axons

The white matter (W) in the cerebrum can be delineated with a dye with an affinity for myelin. The outer cortex (C - gray matter) is composed of nerve cells and does not contain myelin
White matter in the cerebellum is located centrally. Neuron cell bodies are located in the complex folds (folia) and stained purple in this section. (Folding optimises space in this area of the brain)

Question 12

CNS white matter

Answer 12

white matter (axons; many myelinated)

Question 13

CNS grey matter

Answer 13

grey matter (neuron cell bodies, dendrites and axons)

Question 14

White matter in cerebrum

Answer 14

The white matter (W) in the cerebrum can be delineated with a dye with an affinity for myelin. The outer cortex (C - gray matter) is composed of nerve cells and does not contain myelin

Question 15

White matter in the cerebellum

Answer 15

White matter in the cerebellum is located centrally. Neuron cell bodies are located in the complex folds (folia) and stained purple in this section. (Folding optimises space in this area of the brain)

Question 16

Oligodendrocytes

Answer 16

CNS equivalent of a Schwann cell
Myelinate axons
Each oligodendrocyte can myelinated many different axons and therefore each axon will be myelinated by many different oligodendrocytes

Question 17

Astrocytes

Answer 17

Provide mechanical support (also form part of the blood brain barrier)
Maintain microenvironment

Question 18

Microglia

Answer 18

Specialised immunological cells of the CNS

Fight infections in the brain

Question 19

Ependymal cells

Answer 19

Ciliated cuboidal epithelial cells which line the cavities of the brain and spinal cord

Question 20

Peripheral nerves

Answer 20

A nerve consists of one or more bundles of nerve fibres called fascicles
Axons inside the fascicles are surrounded by collagenous support tissue called endoneurium
The fascicles are enclosed in dense collagenous tissue called perineurium
The fascicles are bound together by loose collagenous tissue called epineurium
Note = Schwann cells are the main support cells of the PNS

Question 21

Fascicles

Answer 21

A nerve consists of one or more bundles of nerve fibres called fascicles

Question 22

Endoneurium

Answer 22

Axons inside the fascicles are surrounded by collagenous support tissue called endoneurium

Question 23

PErineurium

Answer 23

The fascicles are enclosed in dense collagenous tissue called perineurium

Question 24

Epineurium

Answer 24

The fascicles are bound together by loose collagenous tissue called epineurium

Question 25

Main support cell of the PNS

Answer 25

schwann cell

Question 26

Myelinated and non-myelinated fibres

Answer 26

Peripheral nervous system axons are enveloped by Schwann cells, providing structural and metabolic support.

Large and small diameter fibres differ in the degree to which they are enveloped:
Small – non-myelinated
Large – myelinated

Question 27

Non-myelinated nerves

Answer 27

Small diameter axons of the autonomic system and small pain fibres are simply enveloped by the cytoplasm of Schwann cells.
Enveloped by the cytoplasm that effectively closes back over and almost connects back with itself in something known as the mesaxon

Question 28

Myelinated nerves

Answer 28

The axon is invaginated into the Schwann cell cytoplasm
The outer membrane of the Schwann cell fuses to form a mesaxon

The mesaxon rotates around the axon – wrapping the axon in concentric layers of membrane = myelin sheath
More complex process for myelinated nerves, Schwann cells envelops an axon and the plasma membrane fuses to get the meson and then after fusion the plasma membrane then wraps around in concentric circles until you get a repeated layering of plasma membrane around the axon and at some point the cytoplasm can effectively be removed and then you basically just have rewearing layers of the plasma membrane wrapped around the axon

Question 29

Nodes of Ranvier

Answer 29

Each Swann cell covers only part of an axon
Each axon in the PNS is not wrapped around by one Schwann cell we actually have many different Schwann cells that are wrapped around a particular part of the axon

Gaps where axons are not myelinated: Nodes of Ranvier

Important in signal conduction along axon
Myelin provides insulation to ensure that signals move fast along the axon and nodes of ranvier are important in conveying the signal as this is where you find the ion channels

Also found in CNS with gaps in oligodendrocyte myelination

Question 30

The resting membrane potential

Answer 30

Having an RMP means that they can propagate APs
An electrical potential exists across the plasma membrane of all cells. The fluid inside the cell has an excess of negative charges and the fluid outside the cell an excess of positive charges.

Non-myelinated nerves are slower to conduct an action potential
They have ion channels all the way along whereas in myelinated fibres they just have ion channels in the nodes so are much quicker

Action potential of myelinated nerves quickly jumps between nodes of Ranvier
Myelination speeds up conduction velocity!

Question 31

Multiple sclerosis

Answer 31

Autoimmune nervous system disease where immune system attacks the myelin of the CNS.

Slows down or blocks messages between the brain and the body….Causes:
Visual disturbances
Muscle weakness
Trouble with coordination and balance, numbness, prickling (“pins and needles“)
Thinking and memory problems

The cause is unknown. There is no cure for MS.
Loss of myelination in various areas of the brain and the spinal cord which is therefore responsible for the large amount of disease features

Question 32

Guillain-Barre syndrome

Answer 32

80% of people make a full recovery

Autoimmune nervous system disease where immune system attacks the myelin of the PNS.
Loss of myelination in the PNS so get symptoms/disease features that are consistent with the loss of communication therefore tingling of hands etc

Tingling in hands and feet
Progressing weakness of limbs and respiratory muscles
Effects on autonomic nervous system lead to altered heart rate and blood pressure.
Cause unknown but usually associated with earlier infection.

Question 33

Synapses

Answer 33

are specialised intercellular junctions which link neurons to each other and to muscles
Synaptic cleft present which is where neurotransmitters are released into so that they can be detected by ligand gated ion channels on the post synaptic cell which can continue the signal to that postsynaptic cell

Question 34

Synaptic transmission at chemical synapses

Answer 34

Propagating axon terminates at the terminal bouton. (Axon giving the information and it terminates at a special structure called the terminal bouton)
Action potential from propagating axon elicits release of neurotransmitter from synaptic vesicles into synaptic cleft
Neurotransmitter diffuses across synaptic cleft and stimulates receptor on the postsynaptic membrane
This stimulates a response, usually an action potential, in the effector cell
Neurotransmitters include: noradrenaline, glutamate, dopamine, acetyl-choline, serotonin

Question 35

neurotransmitter disorders

Answer 35

Neurotransmitters synthesised via biochemical pathways
Loss of enzyme GTP cyclohydrolase 1 leads to deficiency in several neurotransmitters

GTPCH deficiency
Early onset (4-5 months): Intellectual disability Convulsions
Irritability
Hypersalivation
Difficulty breathing

Treatment with neurotransmitter precursors (to regain production of the lost neurotransmitter/s)
Other deficiencies recapitulate effects

Question 36

Motor neurons and NMJ

Answer 36

Neuromuscular junction is the synapse between motor neurons and muscle fibre
Neuromuscular junction also known as motor end plate
Specialisations from other synapses
One motor neuron can divide into many branches each ending in a neuromuscular junction - one neuron may innervate thousands of muscle fibres

Question 37

NMJ

Answer 37

Neuromuscular junction is the synapse between motor neurons and muscle fibre
Neuromuscular junction also known as motor end plate

Question 38

Motor neurons

Answer 38

Motor unit: Motor neuron and connected skeletal muscle fibers
Lots fibres = power
Fewer fibres = endurance ( activate different sets of motor units at different times to allow for the whole muscle to be partially contracted at different times)
Motor neurons branching to innervate a number of muscle fibres

Question 39

Motor unit

Answer 39

Motor unit: Motor neuron and connected skeletal muscle fibers

Question 40

Neuromuscular junction motor end plate process

Answer 40

ACh released from synaptic vesicles

Binds to nicotinic ion channels that cause membrane depolarisation

Secondary synaptic cleft causd by folding

NMJ occupies a recess on the muscle surface - sole plate

Secondary synaptic cleft allows us to have lots and lots of receptors/ion channels for the neurotransmitter so this means that there are lots of different ion channels so the membrane can be depolarised very quickly

Question 41

Muscle fibre action potential

Answer 41

ACh released in response to AP in the motor neuron

Binds to nicotinic receptors, allowing depolarisation of sarcolemma

Action potential along sarcolemma

Into the T tubule

Calcium is released by the sarcoplasmic reticulum and it initiates muscle contraction

Question 42

Myasthenia gravis

Answer 42

Autoimmune disease – body produces antibodies to nicotinic receptor.
Lost the ability of the nicotinic receptors to respond to ACh

Binding of acetylcholine is therefore blocked and muscle activation is inhibited.

Most commonly affected muscles: eyes, face, those associated with swallowing.
All of these are dependent on skeletal muscle fibres

Acetylcholinesterase inhibitors alleviate symptoms
This enzyme taks back up ACh out of the synaptic cleft

Immune suppressors can also help

Question 43

Injecting Botox to eliminate skin wrinkles

Answer 43

Botulinum toxin A regulates ACh release from nerve terminals and thus selectively inhibits the underlying muscles ability to contract.
Causes chronic contraction of certain skeletal muscle fibres in the face to be inhibited

Existing lines and furrows are thus smoothed.

If too much is injected patient can end up with droopy eyelid muscles for weeks.